Multi-layer precision coatings, such as those used small optical systems, are commonly produced in PVD or CVD systems. Many of these coatings are applied to substrates formed of thin flat disks, which are subsequently cut to size.
The coating flux from a source in a PVD or a CVD system are relatively stable; however, they have a spatial distribution that can lead to deposited films with non-uniform thickness, if the substrates remain stationary. To improve uniformity, the geometrical relationship between the source and substrate must be selected appropriately. Good results have been observed when the substrate is rotated about an axis perpendicular to the plane of the surface to be coated, and in particular when multiple substrates have been mounted on multiple spindles in a planetary configuration.
For repeatable thickness control and low defect count the substrate holders must locate the substrate very precisely relative to the other motion elements, and must hold the substrate securely to prevent sliding motion, which could generate particulate contamination under acceleration or temperature changes. Furthermore, the surface of the tooling, (the jig lip) which supports the substrate, must be polished, and must be very thin to prevent shadowing of coating flux arriving at non-normal incidence. The rear surface of the substrate must be shielded to prevent stray coating material from becoming deposited thereon.
To meet these demanding requirements substrate holders for high volume production are typically made of stainless, or even hardened stainless, steel, which is relatively expensive and difficult to maintain. Surfaces adjacent to the substrate are exposed to the same coating flux as the substrate, e.g. multi-layers of hard metal oxides. Unfortunately, the coating on the substrate holders builds up with each coating run and, if not removed, will flake into abrasive particles, which can cause damage to subsequent products. Cleaning the substrate holders usually requires grit blasting or very aggressive chemical etching, which must be repeated every few coating runs or, if very few defects are required, after each coating run. Even hardened steel substrate holders wear out rapidly, resulting in that the substrate holders and the maintenance thereof are a significant cost in the coating process, and a major source of particulate generated defects.
Conventional planetary gear coating systems, such as the one disclosed in U.S. Pat. No. 5,106,346, issued Apr. 21, 1992 to Stefan Locher et al, includes a large rotating platform with several individual spindles (planets) rotatable thereon disposed within a sealed vacuum chamber. Unfortunately, each substrate holder must be connected to a mounting flange on each spindle using mechanical fasteners, e.g. bolts, requiring manual replacement. Not only do these mechanical systems require extra manual labor, they are more susceptible to misalignment caused by changes in temperature and pressure.
In order to isolate as much of the bearing and gear structure as possible from the vacuum chamber, Hurwitt et al disclosed a planetary gear coating system in U.S. Pat. No. 5,795,448 issued Aug. 18, 1998, which includes a magnetic link in the shaft of each spindle. The substrate holders are not suspended over the cathodes, and still require mechanical fasteners for attachment to the mounting flanges of the spindles.
The coating system, disclosed in U.S. Pat. No. 6,464,825 issued Oct. 15, 2002 to Shinozaki, includes a robotic arm traveling between a pressurized loading/unloading chamber and the main vacuum chamber to minimize the amount dust entering the main vacuum chamber. The Shinozaki system also includes a magnetic rotational drive and a magnetic levitating member to minimize particulate generation caused by interacting mechanical elements. However, Shinozaki discloses a single rotating platform with a complicated levitating platform and electromagnets that totally surround the substrate holder. Unfortunately, this approach would be impossible to implement in a planetary gear coating system, as it is very difficult to deliver power separately to individual rotating substrate holders in a planetary system, while operating in a vacuum and at elevated temperatures.
An object of the present invention is to overcome the shortcomings of the prior art by providing a substrate holder with a disposable cover, which is easily removable from the base, and can be easily dismantled to remove the substrate therefrom.